1. Field of the Invention
This invention relates generally to chemical-mechanical polishing in the processing of oxides and metals. This invention relates particularly to CMP processing of semiconductor wafers.
2. Description of the Related Art
Chemical Mechanical Polishing (CMP) is a chemical reaction accelerated by polishing action. It should be understood that CMP does not involve grinding or scraping physical ablation. CMP of oxides, particularly SiO.sub.2, is generally accomplished with high pH solutions, such as an aqueous solution of potassium hydroxide, with colloidal suspensions of particles, such as SiO.sub.2 or cesium oxides. The high pH solution is required to provide a strong lewis acid to hydrate the oxide. Even with the strong lewis acid, the reaction will not proceed at an appreciable reaction rate at room temperature. The activation energy is supplied by the polishing action. The colloidal particles deflect and strain the surface of the oxide in what is called the traveling Hertzian indenter.
The reaction rate and uniformity are controlled by various process parameters such as wafer rotation, downward pressure, curvature of wafer/backplane, polish table rotation, nape of polish pads, viscosity, maintenance of equipment and conditioning of pads. All of these items essentially affect surface reaction, boundary layer diffusion, and bulk diffusion.
The CMP process is difficult to control to the tolerances required for semiconductor manufacture. Even though the reaction rate is acceptably fast, large amounts of energy are required in CMP processing. Very large electrical motors are required to provide stable drives to the various disks and platens that are typically used in CMP processing. Satisfactorily balancing the reaction rates and diffusion rates, in particular, is very difficult. For example, a curved backplane is used to supply more downward pressure, i.e. activation energy, in the center of the wafer to balance the reduced linear travel of the rotating backplane and reduced diffusion rate in the center of the wafer.
CMP of metals is generally accomplished in low pH acids. The reaction of the acid with the metals is very fast. Therefore, the surface of the metal is oxidized to retard the reaction. Oxidation of the metal surface can be accomplished by adding an oxidant such as Hydrogen peroxide before removal of the oxided metal by the CMP action. A colloidal suspension of particles deflects and strains the surface of the oxide to provide activation energy in the traveling Hertzian indenter.
Metals are often removed as part of a manufacturing flow called Damascene. In this process flow grooves or trenches are first formed in an insulating layer. A blanket coating of metal is applied to the trenches. Then the metal in all areas except the trenches is removed by CMP. This leaves lines of metal interconnect separated by insulators. One problem is the balance required to remove all excess metal in the insulating areas without gouging out the metal in the interconnect areas.
The rapid metal reaction must be balanced with the slower oxide reaction, the boundary diffusion of reactants and products, and the bulk diffusion of reactants.